Due to the nature of bags (e.g., having non-square sides), stacking pallets can result in the bags being stacked lopsided on the pallet. This is particularly true when the bags (e.g., bags of mulch, or anything else sold in a bag) are stacked automatically by robot, because the robot simply drops the bags onto the pallet.
In order to better shape the stacked bags, pallets have been provided in the form of a fixed box, and the bags are dropped into the box. These pallets are more expensive (i.e., the cardboard walls), or the pallet then has to be lifted up out of the box.